my first advice..
do not overcapitalise on the car.
 
2. GT35R is very fun at 1.7 Bar
 

You're right you don't need the valves. I ended up doing it because my original valves were bent, so i changed them anyway.

To be fair.....it is likely only an isolated case(s). But it's another aftermarket item that can fail when Toyota valves are proven. One thing I've learned over the years, is to avoid changing items just for the sake of it (but we all do to some extent).
 
2. Richard can't say he never had a problem afterwards because the car was sold shortly after it was repaired. Many of the big HP US 3SGTE's have had valve issues for numerous reasons.